The present invention relates generally to the conversion of modulated light to electrical signals and more particularly, to reception of broadband radio-frequency (R-F) modulated signals including signals found in multichannel amplitude modulated (AM) television distribution systems transmitted through fiber optic cable.
Serviceable optical receivers now exist for fiber-optic audio and digital communications systems. However, there is a lack of inexpensive, low distortion, low noise optical receivers for R-F fiber-optic cable transmitted signals. In the U.S. broadband cable television signals are amplitude modulated and include the frequencies from 50 MHz to 550 MHz. Other systems use frequencies as high as 860 MHz, and systems have been proposed for frequencies of up to 1 GHz. As alternatives to transmission of AM television signals through fiber-optic cable, frequency modulated (FM) and digitally encoded television signals have been demonstrated. However, such alternatives to AM transmission are not competitive with existing wire cable television systems because of the equipment needed to encode and decode the television signal. AM transmission of television through fiber-optic cable requires an optical receiver which can amplify the received signal with very low distortion and which adds a minimum amount of noise.
Prior art optical receivers have specially designed low noise amplifiers connected directly to photodiodes. However, such optical receivers are used primarily for transmission of digital communication and have more inherent distortion than is permissible for the transmission of broadband AM signals.
This invention provides an inexpensive practical optical receiver which operates with broadband signals over the R-F frequencies of from 10 MHz to more than 1 GHz. The invention overcomes the noise and distortion problems associated with fiber-optic cable receivers and is designed to operate with low cost, commercially available amplifiers.